[MERGE/RFC] no lock for 'is_shared'

[Martin Pool]

Martin Pool has voted approve.
Status is now: Approved
Comment:
This seems reasonable to me.  It is technically an API change and should 
be in NEWS.

It would be interesting and not so hard to add a decorator that 
automatically takes a lock, but says that autolocking for this method is 
deprecated and the caller should do it.  I don't think it's needed now 
but if we make a push at some time to remove these we should add it.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C48850D4D.4020907%40arbash-meinel.com%3E